This invention relates to a system and a method for damping vibrations of structures, particularly although not exclusively, those structures undergoing a machining operation.
During a machining operation, for instance milling a metallic component, it is common for chatter to occur if the tooling or workpiece are of insufficient rigidity. Chatter is the vibration of the milling tool relative to the workpiece which results in either a reduction in the quality of the surface finish being machined or an increase in the machining process time where a better surface finish is required. In the manufacture of aero-engine blisks this is of particular importance as the tough nature of the material, titanium, to be machined and the flexibility and low inherent damping of the workpiece severely curtails machining rates.
Current damping techniques include magnetostrictive actuators as disclosed in xe2x80x9cVibration suppression in slender composite beams using magnetostrictive actuationxe2x80x9d, Journal of Aeronautical Society of India, vol. 48, no. 2, 1996. However, this technique is disadvantaged by being an active system, requiring a power supply, wiring to induce a magnetic field and by incurring a weight penalty. Another damping technique uses piezoelectric shunts, as disclosed in xe2x80x9cNon-linear piezoelectric vibration absorbersxe2x80x9d, Smart Materials and Structures, vol. 5, no. 5, 1996. Such shunts essentially convert mechanical strain, from vibrations, into electrical charge which is then dissipated via an electrical network thus attenuating vibration. This system is disadvantaged by requiring wire connections and added weight. A further damping technique uses visco-elastic material as disclosed in xe2x80x9cVibration dampingxe2x80x9d, John Wiley and Sons, 1985, but suffers from a weight penalty and the difficulty of maintaining optimal environmental conditions for required performance.
It is the object of the present invention to provide a vibration damping system and a method for damping vibrations of vibrating structures which may accommodate irregular workpiece and tooling geometries, may operate in extreme environmental conditions, may be used where access is limited and which has a reduced weight penalty.
According to the present invention, there is provided a vibration damping system wherein the system comprises a magnetism generating medium and a magnetism energy dissipating medium whereby, in use, vibration of the magnetism generating medium generates a magnetic field, the magnetism generating medium and the magnetism energy dissipating medium being so disposed with respect to each other that the magnetic field is then dissipated by the magnetism energy dissipating medium thereby damping the vibrations of the magnetism generating medium.
Alternatively, the system comprises a body, the magnetism generating medium is applied to the body, so that, in use, when the body vibrates a magnetic field is generated by the magnetism generating medium, the magnetism generating medium and the magnetism energy dissipating medium being so disposed with respect to each other that the magnetic field is then dissipated by the magnetism energy dissipating medium thereby damping the vibrations of the body.
Preferably, the vibration damping system comprises a secondary structure, the magnetism energy dissipating medium is applied to the secondary structure and the secondary structure, in use, is in proximity to the body such that the vibrations of the body are damped.
Preferably, the secondary structure comprises magnetism energy dissipating medium and, in use, is in proximity to the body such that the vibrations of the body are damped. Alternatively, the magnetism energy dissipating medium is applied to the magnetism generating medium.
Preferably, the magnetism generating medium comprises magnetostrictive material. Alternatively, the magnetism generating medium comprises a matrix and a plurality of magnetostrictive particles dispersed within the matrix.
Preferably, the magnetism energy dissipating medium comprises a matrix and a plurality of magnetostrictive particles dispersed within the matrix.
Preferably, the magnetostrictive particles and the magnetostrictive material each comprise 30% Terbium, 70% Dysprosium and trace Iron. Alternatively, the magnetostrictive particles and the magnetostrictive material each comprise any one or more of the magnetic transition elements. Alternatively, the magnetostrictive particles and the magnetostrictive material each comprise any one or more of the rare earth materials. Alternatively, the magnetostrictive particles and the magnetostrictive material each comprise any one or more of the rare earth materials and any one or more of the magnetic transition elements.
Preferably the magnetism energy dissipating matrix comprises polyurethane. Alternatively, the magnetism energy dissipating medium comprises a matrix, the matrix comprises a visco-elastic magnetism energy dissipating matrix.
Preferably, the magnetism energy dissipating medium includes a visco-elastic magnetism energy dissipating matrix and the magnetism generating medium includes a binder matrix, the binder matrix having an elastic modulus greater than the elastic modulus of the visco-elastic dissipating matrix.
Preferably, each of the magnetostrictive particles has substantially the same magnetic orientation as the remainder thereof. Alternatively, each of the magnetostrictive particles is oriented substantially in the direction of the magnetic field.
Alternatively, the magnetostrictive particles are each of the same size.
Preferably, the vibrations imposed on the magnetism generating medium are derived from a machining process.
Alternatively, the vibrations imposed on the magnetism generating medium are derived from an acoustic source.
Preferably, the body is any one of the primary structure, the machine, the tool or any industrially manufactured item which undergoes some form of machining process. Alternatively, the body is a component of a gas turbine engine. Alternatively, the body is a component of an aircraft. Alternatively, the body is a vibrating rotating body.
Preferably, the magnetism generating medium is any one of the primary structure, the machine, the tool or any industrially manufactured item which undergoes some form of machining process. Alternatively, the magnetism generating medium is a component of a gas turbine engine. Alternatively, the magnetism generating medium is a component of an aircraft. Alternatively, the magnetism generating medium is a vibrating rotating body.
Preferably, a means for cooling the secondary structure is provided. Preferably, a means for cooling the magnetism energy dissipating coating is provided. Preferably, the means for cooling comprises any conventional cooling system as known in the art.
Preferably, a method of damping vibrations of a body wherein the method comprises the steps of:
(i) applying a magnetism generating medium to a body;
(ii) applying a magnetism energy dissipating medium to a secondary structure;
(iii) disposing the secondary structure in proximity to the body so that the magnetism generating coating and the magnetism energy dissipating medium are so disposed with respect to each other that the magnetic field is then dissipated by the magnetism energy dissipating medium thereby damping the vibrations of the body.
Preferably, the method of damping vibrations of the body comprises the further step of cooling the magnetism energy dissipating medium to optimise operational temperature of the magnetism energy dissipating medium.
Preferably, the method of damping vibrations of the body comprises the further step of cooling the secondary structure.